Thermoresponsive colloidal microgels and polymeric solutions at rest and under shear
The structure of temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgels was investigated by means of small-angle neutron scattering (SANS). A direct modeling expression for the scattering intensity was derived. The influences of temperature, cross-linking density and particle size on the structure were revealed by the radial density profiles and clearly showed that the segment density in the swollen state was not homogeneous but gradually decayed at the surface. The density profile revealed a box profile only when the particles were collapsed at elevated temperatures. The overall particle size and the correlation length of the concentration fluctuations of the internal polymer network decreased with concentration revealing the increasing compression of the spheres. The interaction potential did not change significantly between 25°C and 32°C. Microgels with different degrees of cross-linking and particle size resembled true hard sphere behavior up to effective volume fractions of <0.35. At higher effective volume fractions strong deviations from true hard spheres were observed. At temperatures well above the LCST the interaction potential became strongly attractive. The collapsed microgel spheres formed aggregates consisting of flocculated particles without significant long-range order. Rheo-SANS experiments revealed that the shear-induced particle arrangements strongly depend on the interaction potential. When the interaction potential was repulsive at temperatures below the LCST, no significant deformation of the swollen PNiPAM particles was observed. Shear-induced ordering was found resulting in the formation of two dimensional hexagonal close packed layers that aligned along the flow direction giving rise to shear thinning. At temperatures near the LCST, when the particle interaction potential is not yet strongly attractive, shear flow induced the collapse of an individual particle. A so-called butterfly scattering pattern indicated the shear-induced enhancement of concentration fluctuations along the flow direction leading to solvent being squeezed out of the particles. The influence of shear flow on the phase separation of PNiPAM microgels was investigated by means of rheo-turbidity and rheo-SANS and compared to the behavior of linear PNiPAM macromolecules. The rheological behavior of concentrated microgel suspensions depended strongly on temperature, but flow and viscoelastic properties of concentrated solutions of the linear polymer were not significantly affected by temperature changes. Shear induced phase separation was observed for both polymer architectures, although the viscoelastic properties of the two systems have different structural origins. For solutions of aqueous linear chain PNiPAM in the semi-dilute regime at different shear rates the existence of a threshold shear stress was observed and the phase separation process became faster with increasing stress. The two dimensional scattering patterns remained isotropic even during the phase separation process and the correlation length increased. The influence of shear flow on the phase separation process has apparently an analogous effect as a temperature increase.